The present invention relates to an optically anisotropic cellulose ester film. The invention also relates to an optical compensatory sheet, an ellipsoidal polarizing plate and a liquid crystal display.
A liquid crystal display generally has a liquid crystal cell, a polarizing element and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing elements are provided on both sides of the liquid crystal cell, and one or two optical compensatory sheets are placed between the liquid crystal cell and the polarizing element. On the other hand, a display of reflection type comprises a reflection board, a liquid crystal cell, one optical compensatory sheet and one polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates, and the electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. According to alignment of the rod-like liquid crystal molecules in the cell, various display modes are proposed. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include TN mode and HAN (hybrid aligned nematic) mode.
The polarizing element generally comprises two transparent protective films provided on both sides of a polarizing film.
The optical compensatory sheet is widely used in various liquid crystal displays because it prevents the displayed image from undesirable coloring and enlarges a viewing angle. As the optical compensatory sheet, a stretched birefringent film has been used.
In place of the stretched birefringent film, an optical compensatory sheet comprising an optically anisotropic layer on a transparent substrate has been proposed. The optically anisotropic layer is formed by aligning liquid crystal molecules (in particular, discotic liquid crystal molecules) and fixing the aligned molecules. For preparing the optically anisotropic layer, a liquid crystal compound having a polymerizable group is generally used and the alignment of the molecules is fixed by polymerization reaction. The liquid crystal molecules usually have large birefringence, and they also have various alignment forms. Accordingly, an optical compensatory sheet obtained by using the liquid crystal molecules has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film.
The optical characteristic of the compensatory sheet is determined according to the aforementioned display mode of the liquid crystal cell. If liquid crystal molecules, particularly discotic liquid crystal molecules are used, various compensatory sheets suitable for various display modes can be produced.
Actually, according to various display modes, various optical compensatory sheets comprising discotic liquid crystal molecules have been proposed. The optical compensatory sheet for a TN mode is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. Japanese Patent Provisional Publication No. 10(1998)-54982 discloses the compensatory sheet for an IPS or FLC mode. The sheet for an OCB or HAN mode is described in U.S. Pat. No. 5,805,253 and International Patent No. WO96/37804. Japanese Patent Provisional Publication No. 9(1997)-26572 discloses the sheet for a STN mode. The optical compensatory sheet for a VA mode is disclosed in U.S. Pat. No. 2,866,372.
Although liquid crystal molecules can be oriented in various alignments, there is a case that the optical anisotropy of the aligned molecules can not fully compensate the liquid crystal cell by itself. For that case, U.S. Pat. No. 5,646,703 proposes to use an optical anisotropic film as the transparent support to compensate the liquid crystal cell in cooperation with the optical anisotropy of the aligned liquid crystal molecules. As the optical anisotropic support, a stretched film of synthetic polymer has been practically used.
However, a conventional optical anisotropic support of synthetic polymer film does not satisfyingly function as a support. Further, that film is difficult to use for an ellipsoidal polarizing plate in which the optical compensatory sheet and the polarizing element are unified.
The applicants have tried to use a cellulose ester film where the film should have an optical anisotropy (a high retardation value). As a support, the cellulose ester film is superior to a stretched film of a synthetic polymer. Therefore, if a cellulose ester film has a high optical anisotropy (a high retardation value), the cellulose ester film can be used as an optically anisotropic support of an optical compensatory sheet.
However, a cellulose ester film having a low retardation value has been considered preferable according to prior art. Therefore, a method of increasing the retardation value of the cellulose ester film has been scarcely studied, while a method of decreasing the retardation value has been intensively studied.
The applicants have studied the method of increasing the retardation value of the cellulose ester film, and have succeeded in obtaining a cellulose ester film having a high retardation value.
An object of the present invention is to provide a cellulose ester film having a high retardation value.
Another object of the present invention is to provide an optical compensatory sheet comprising a cellulose ester film having a high retardation value.
A further object of the invention is to provide an ellipsoidal polarizing plate comprising a cellulose ester film having a high retardation value.
A furthermore object of the invention is to provide a liquid crystal display comprising an optically anisotropic transparent support.
The present invention provides a cellulose ester film containing a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight parts of cellulose ester, wherein the film has a Rth550 retardation value defined by the following formula in the range of 60 to 1,000 nm:
Rth550=[{(nx+ny)/2}xe2x88x92nz]xc3x97d
in which each of nx and ny is the principal refractive index measured by light of 550 nm in plane of the film; nz is the principal refractive index measured by light of 550 nm along the thickness direction of the film; and d is the thickness of the film.
The invention also provides an optical compensatory sheet consisting of a cellulose ester film, wherein the film contains a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight parts of cellulose ester, and wherein the film has a Rth550 retardation value in the range of 60 to 1,000 nm.
The invention further provides an optical compensatory sheet comprising a cellulose ester film and an optically anisotropic layer comprising liquid crystal molecules, wherein the film contains a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight parts of cellulose ester, and wherein the film has a Rth550 retardation value in the range of 60 to 1,000 nm.
The invention furthermore provides an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing film, a transparent support and an optically anisotropic layer comprising liquid crystal molecules in this order, wherein the transparent support is a cellulose ester film containing a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight, parts of cellulose ester, and wherein the film has a Rth550 retardation value in the range of 60 to 1,000 nm.
The invention still further provides a liquid crystal display comprising a liquid crystal cell and two polarizing elements arranged on both sides of the liquid crystal cell, wherein at least one of the polarizing elements is an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing film, a transparent support and an optically anisotropic layer containing liquid crystal molecules in this order, wherein the transparent support is a cellulose ester film containing a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight parts of cellulose ester, and wherein the film has a Rth550 retardation value in the range of 60 to 1,000 nm.
The invention still furthermore provides a liquid crystal display comprising a reflection board, a liquid crystal cell and a polarizing element in this order, wherein the polarizing element is an ellipsoidal polarizing plate comprising a transparent protective film, a polarizing film, a transparent support and an optically anisotropic layer containing liquid crystal molecules in this order, wherein the transparent support is a cellulose ester film containing a discotic compound in an amount of 0.01 to 20 weight parts based on 100 weight parts of cellulose ester, and wherein the film has a Rth550 retardation value in the range of 60 to 1,000 nm.
The applicants have found that a discotic compound increases the retardation value of the cellulose ester film. If the discotic compound is added in an amount of 0.01 to 20 weight parts based on 100 weight parts of the cellulose ester, the film having a Rth550 retardation value of 60 to 1,000 nm is obtained. The film having such a high retardation value can be directly used as an optical compensatory sheet for a liquid crystal display. Further, the film is used as a support for an optical compensatory sheet in which an optically anisotropic layer containing discotic liquid crystal molecules is provided on the support.
Even if a rod-like compound (in which two or more aromatic rings are linearly connected) is used in place of the discotic compound, the cellulose ester film having a high retardation value can be obtained. However, the discotic compound increases the retardation value more than the rod-like compound, and hence a satisfying retardation value can be given by the discotic compound even in a relatively small amount. Further, the discotic compound hardly deposits (bleeds out) on the surface of the film, as compared with the rod-like compound. Accordingly, the cellulose ester film having a high retardation value can be obtained by the discotic compound without bleeding out.
The cellulose ester film having a high retardation value satisfyingly functions as a support, and hence is advantageously used as an optically anisotropic transparent support of an optical compensatory sheet. Further, this cellulose ester film protects the polarizing film well, and hence is advantageously used as optically anisotropic transparent support of an ellipsoidal polarizing plate of unified type.
The ellipsoidal polarizing plate comprising a transparent protective film, a polarizing film, an optically anisotropic transparent support of the cellulose ester film having a high retardation value and an optically anisotropic layer containing liquid crystal molecules is advantageously used for a liquid crystal display of a TN (twisted nematic), VA (vertically aligned), OCB (optically compensatory bend) or HAN (hybrid aligned nematic) mode.